1. Field of the Invention
The present invention relates to a mobile communication terminal such as a portable telephone, a portable information terminal (e.g., PDA: Personal Digital Assistants) or the like that is provided with a pair of antennas.
2. Description of the Related Art
Generally, a small mobile communication terminal such as a portable telephone or a portable information terminal is provided with a pair of antennas for the purpose of down-sizing the terminal and maintaining a good reception sensitivity.
One of the antennas, or the first antenna, is typically a helical antenna that is rigidly secured to the top of the terminal cabinet. On the other hand, the other antenna, or the second antenna, is typically a rod antenna that is extendable and contained in the terminal cabinet.
In an idle state, the second antenna is pulled in the terminal cabinet and the incoming call signal is received by means of the first antenna. On the other hand, in a communication state, the second antenna is extended upward from the terminal cabinet and used.
With the above described arrangement, only the first antenna is projecting from the terminal cabinet in the idle state so that the terminal provides a good portability.
Meanwhile, in recent years, there have been developed various mobile communication terminals that are equipped with a mail communication feature, a video communication feature involving the use of a camera and so on in addition to the speech communication feature. Such a mobile communication terminal is used not only for speech communication where the terminal is put to one of the ears of the user but also for data communication where the user operates the terminal, watching the image on its screen.
Therefore, the antenna that is being in service for communication is required to be relatively free from the influence of the user's head and to improve the antenna gain for speech communication. On the other hand, the antenna that is being in service for communication is required to be non-directional for data communication and to maintain the antenna gain on a stable basis.
When the characteristics of mobile communication terminals marketed in recent years are examined from the viewpoint of fulfilling the above requirements, it will be seen that the antenna that is being used for data communication fulfills the requirement for that purpose but the performance of the antenna that is being used for speech communication can be degraded under the influence of the user's head.
Particularly, the radiation element of the helical antenna that becomes active when the rod antenna is pulled in can be shaded by the user's head to a large extent. Therefore, it is apt to be significantly influenced by the user's head and hence can hardly maintain the antenna gain to a desired degree.
Additionally, it is well known that the input impedance of the antenna fluctuates as viewed from the feeding point of the antenna to make it impossible to maintain desired frequency characteristics when the mobile communication terminal is held by hand or brought close to the ear. Therefore, the antennas need to be designed by taking these influences into consideration.
Particularly, in the case of next generation mobile communication systems such as IMT2000, the radio frequency band is broadened by several times to make it highly difficult to design antennas if compared with that of conventional systems. More specifically, when the transmission frequency and the reception frequency is separated from each other by about 10 percent in terms of band ratio, it is extremely difficult to design the antennas if the conventional combination of a rod antenna and a helical antenna is used. Because the input impedance of the antenna fluctuates due to the difference between the transmission frequency and the reception frequency. If the antennas are not designed properly, the reception sensitivity will become poor particularly in an idle state and the quality of communication will be remarkably degraded for speech communication and data communication.
Generally, in the case of mobile communication terminals such as a portable telephone, not only the antennas but also the terminal cabinet are active as radiator to radiate a radio wave from the terminal. Therefore, the size of the cabinet is an important design parameter for designing the antennas of the terminal. Additionally, as a result of the use of higher radio frequencies in recent years, the cabinet has become large relative to the wavelength and hence the unbalanced current of the antenna is induced into the cabinet in a complex manner. This situation provides a large factor that makes the design of the antennas even more difficult.
Various proposals have been made to suppress the unbalanced current that is induced into the cabinet in order to avoid the problem. According to a proposal, for example, the terminal cabinet is notched to suppress the unbalanced current induced into the bottom side of the cabinet (see, for example, Japanese Patent Application KOKAI Publication No. 5-327527). According to another proposal, the substrate is provided with a stub to suppress the current induced into the bottom side of the cabinet.
While these proposals are very effective, forming such a notch or stub may not be feasible for mobile communication terminals such as a portable telephone and so on that are compact and densely mounted with components.